Proton Mass Mystery Could Mean New Physics

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The size of a proton, long thought to be well-understood, may remain a mystery for a while longer, according to physicists.

Speaking on Saturday (April 13) at the April meeting of the American Physical Society, researchers said they need more data to understand why new measurements of proton size don't match old ones.

"The discrepancy is rather severe," said Randolf Pohl, a scientist at the Max Planck Institute of Quantum Optics. The question, Pohl and his colleagues said, is whether the explanation is a boring one -- someone messed up the measurements -- or something that will generate new physics theories.

The Incredible Shrinking Proton

The proton is a positively charged particle in the nucleus of atoms, the building blocks of everything. Years of measurements pegged the proton at 0.8768 femtometers in radius (a femtometer is a millionth of a billionth of a meter).

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But a new method used in 2009 found a different measurement: 0.84087 femtometers, a 4 percent difference in radius.

The previous measurements had used electrons, negatively charged particles that circle the nucleus in a cloud, to determine proton radius. To make the measurement with electrons, researchers can do one of two things. First, they can fire electrons at protons to measure how the electrons are deflected. This electron-scattering method provides insight into the size of the positively charged proton.

An alternative is to try to make the electron move. Electrons zing around the nucleus of an atom, where protons reside, at different levels called orbitals. They can jump from orbital to orbital by increasing or decreasing their energy, which electrons do by losing or gaining an elementary particle of light called a photon. The amount of energy it takes to budge an electron from orbital to orbital tells physicists how much pull the proton has, and thus the proton's size.

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Pohl and his colleagues didn't use electrons at all in their measurements of the proton. Instead, they turned to another negatively charged particle called the muon. The muon is 200 times heavier than an electron, so it orbits the proton 200 times closer. This heft makes it easier for scientists to predict which orbital a muon resides in and thus a much more sensitive measure of proton size.

"The muon is closer to the proton and it has a better view," Pohl said.

Possible Explanations

These sensitive muon measurements are the ones that gave the smaller-than-expected result for the proton radius, a totally unexpected discovery, Pohl said. Now, physicists are racing to explain the discrepancies.

One possibility is that the measurements are simply wrong. Pohl said this "boring explanation" is the most probable, but not all physicists agree.

"I would say it's not the experimental side," said Massachusetts Institute of Technology physicist Jan Bernauer.

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